Good ideas. From what I see, there is no damage cap for cav unless you're ranged.
Horses have always been on an increasing scale of usefulness as you increase the difficulty. I actually severely buffed some of the lighter horses and they still aren't all that great as you can see. It's an interesting prospect to buff mobility, I definitely think speed at the very least should be improved though 55 is just meme tier.
Maneuver is the elephant in the room and is why some horses have such terrible speed. You can do some crazy stuff with a horse once you hit 44 maneuver, but the question is if it's really that bad.
With the bumpslash nerf, I personally think that cav have room to grow in terms of buffs, but it needs to be treated carefully. It will be tough to get other balancers to agree if the proposal is too egregious.
I have been using cav for nearly 5 years as well and even though the playstyle can be dumb, it's still a big performance booster. I think it should be made more fun without improving the performance too much.